1. Field of the Invention
This invention relates generally to the field of superhydrophobic surfaces, and more specifically to the patterning of superhydrophobic surfaces to control the storage, mobility and transport of liquid drops for microfluidic applications.
2. Description of Related Art
The Lotus Effect is named after the lotus plant, and was first used for technical applications by Professor Wilhelm Barthlott from the University of Bonn. The Lotus Effect generally refers to two characteristic properties: superhydrophobicity and self-cleaning, although in some instances, either one of these properties provide the benefits of the Lotus Effect.
Superhydrophobicity is manifested by a water contact angle larger than 150°, while self-cleaning indicates that loose (non-adhered) dirt particles such as dust or soot are picked up by a drop of water as it rolls off the surface, and are thus removed. The superhydrophobicity and self-cleaning properties of a Lotus Effect surface are illustrated in FIGS. 1a and 1b. 
TABLE 1 provides common definitions of liquid/surface phenomena related to water. For example, it will be understood that the values will change with other liquids, such as saline solution, wherein in a low concentration saline solution, there is no appreciable effect, but in higher saline concentrations, the contact angle will be lower. Thus, these definitions are also applicable to liquids with low concentrations of salts and particulates such as those found in normally encountered environmental pollution and biological fluid environments.
TABLE 1Contact AngleHysteresisDescription(degrees)(degrees)Hydrophilic <45—Hydrophobic>45 and <150>10Superhydrophobic>150<10
In general, a Lotus Effect surface arises when both of the following factors are achieved: the surface is covered with low surface free energy materials, and has a very fine structure. Low surface free energy materials provide a relatively high contact angle. The contact angle is a measure of the wettability of a surface with a fluid—water in this case. Readily wettable (hydrophilic) surfaces have relatively small water contact angles, and non-wetting (hydrophobic) surface have relatively large contact angles.
Regarding surface structure, surfaces that are rough tend to be more hydrophobic than smooth surfaces, because air can be trapped in the fine structures, which reduces the contact area between the liquid and the surface, or water and solid. It is recognized that when a water drop is placed on a lotus plant surface, the air entrapped in the nanosurface structures prevents the total wetting of the surface, and only a small part of the surface, such as the tip of the nanostructures, is in contact with the water drop. For the lotus plant leaves, the actual contact area is only 2-3% of a droplet-covered surface. This enlarges the water/air interface while the solid/water interface is minimized. Therefore, the water gains very little energy through adsorption to compensate for any enlargement of its surface. In this situation, spreading does not occur, the water forms a spherical droplet, and the contact angle of the droplet depends almost entirely on the surface tension of the water. The relationship between the surface water contact angle and the surface structural geometry (Wenzel roughness) can be given in Cassie-Bexter equation:cos θA=rf1 cos θY+f1−1  Equation 1
where the parameter r is the ratio of the actual solid-liquid contact area to its vertical projected area (Wenzel roughness factor), θA is the apparent contact angle on the rough surface, and θY is the contact angle on a flat surface as per Young's equation, f1 is the solid surface fraction.
Thus, the contact angle, θ, is a quantitative measure of the wetting of a solid by a liquid. It is defined geometrically as the angle formed by a liquid at the three phase boundary where a liquid, gas and solid intersect.
A low value of contact angle (θ) indicates that the liquid spreads, or wets well, while a high contact angle indicates poor wetting. If the angle θ is less than 90°, the liquid is said to wet the solid. If it is greater than 90°, it is said to be non-wetting. A zero contact angle represents complete wetting.
The difference between the maximum and minimum contact angle values is the contact angle hysteresis. FIG. 2 is a schematic representation of contact angle and angle hysteresis, which helps characterize surface heterogeneity, roughness and mobility. For surfaces which are not homogeneous, there will exist domains on the surface that present barriers to the motion of the contact line. For the case of chemical heterogeneity, these domains represent areas with different contact angles than the surrounding surface. For example, when wetting with water, hydrophobic domains will pin the motion of the contact line as the liquid advances, thus increasing the contact angles. When the water recedes, the hydrophilic domains will hold back the draining motion of the contact line, thus decreasing the contact angle. Thus, when testing with water, advancing angles will be sensitive to the hydrophobic domains, and receding angles will characterize the hydrophilic domains on the surface.
For situations in which surface roughness generates hysteresis, the actual microscopic variations of slope in the surface create the barriers that pin the motion of the contact line and alter the macroscopic contact angles.
Although the Lotus Effect was discovered in plants, it is essentially a physicochemical property rather than a biological property. Therefore, it is possible to mimic the lotus surface structure.
Superhydrophobic properties are desirable for many applications. Recently, there has been an increased interest in the fabrication of superhydrophobic surfaces with high adhesive force. Interestingly, these two seemingly incompatible properties—high advancing contact angle (repulsion force) and high adhesive force (attraction force)—can be combined on a single surface which has been termed as “sticky” superhydrophobic surface.
Currently, paper is more than just a substrate for writing, printing and packaging; recent scientific research has established its potential as an inexpensive, biodegradable, renewable, flexible polymer substrate. Innovative concepts of paper-based devices include transistors, batteries, super-capacitors, MEMS devices, sensors, and lab-on-a-chip (LOC) microfluidic devices.
It is known that superhydrophobic paper/cellulose surfaces can be fabricated with tunability in adhesive force (from extremely sticky to non-sticky or roll-off) by using a two-step plasma enhanced chemical vapor etching/deposition process. Further, there has been an increased interest in investigating paper as a potential candidate for lab-on-a-chip/microfluidics devices and field-effect transistors.
Fabrication of extremely water repellant superhydrophobic paper surfaces has been shown (contact angle (CA)≈166.7±0.9° ; CA hysteresis≈3.4±0.1°) for potential applications in the chemical and biomedical fields via plasma treatment. In defining superhydrophobicity, researchers often focus on the advancing contact angle, but the receding contact angle plays an important role as well, and protocols have been developed to control the adhesion of water drops on paper substrates by tuning the contact angle hysteresis between 149.8±5.8° and 3.5±1.1°, while maintaining the advancing CA above 150°. To distinguish between these substrates, we use the terminology “roll-off superhydrophobic” (CA>150°; hysteresis<10°) for low hysteresis substrates that exhibit the so-called lotus effect, and “sticky superhydrophobic” for substrates with high hysteresis (CA>150°; hysteresis>10°).
In the early stages of their development, LOC microfluidic devices were fabricated with technologies originally developed for the microelectronics industry, in particular photolithography and etching, and thus were fabricated from silicon wafers or glass substrates. Subsequently, researchers began investigating polymers as substrates (especially PDMS) in combination with soft lithography techniques because of the advantages of these substrates over silicon-or glass-based devices: transparency, flexibility, biochemical compatibility and permeability. However, even PDMS-based devices require the use of clean room facilities for the fabrication and incorporation of complex components such as valves, pumps and mixers.
Fluid actuation in these types of devices relies mostly on electrokinetic or pneumatic actuation, which requires an external power source (high voltage power supply, batteries, or compressed gas/vacuum sources). Overall, in spite of breakthrough advances in LOC concepts, most of the devices remain unsuitable for low-tech applications like biomedical diagnostics in developing countries due to the lack of simplicity and affordability.
Paper-based LOC devices (also referred to as lab-on-paper (LOP)) have emerged as a promising alternative technology. For fluid actuation on these devices, one can rely on capillary forces inside the porous paper, and thus avoid external power sources.
In a recent report on the top ten biotechnologies for improving health in developing countries “modified molecular technologies for affordable, simple diagnosis of infectious diseases” were ranked as the number one priority. Another report on the grand challenges for global health ranked the development of technologies to “measure disease and health status accurately and economically in poor countries” first among the top 14 priorities. Due to their affordability and potentially simple fabrication technology, LOP devices may offer improved global availability of medical technology.
In its simplest form, the concept of LOP dates back to the 1950s, when paper-based strips were first used for biomedical diagnostics. However, applications of these LOPs were limited by the fact that they could not perform multiplex analysis: i.e., it was impossible to perform multiple biochemical analyses on a single sample with the same strip. This limitation inspired the fabrication of multiple channels with barriers within a paper substrate, analogous to a microfluidic device, to enable multiplex analysis.
Creation of hydrophilic channels with hydrophobic barrier layers for biochemical assay devices was originally proposed in 1995 and 2003. More recently, this concept has been adapted by using modern photolithography techniques to create hydrophobic photoresist barriers.
This work has since been expanded to three-dimensional LOP devices by layering sheets of patterned paper with perforated barrier tape to guide the exchange of liquids between paper layers. Yet, a disadvantage of these LOPs was the limited flexibility due to the use of rigid photoresists (SU-8 or PMMA), which has been addressed by printing PDMS as a barrier polymer using a desktop plotter, thus creating flexible LOP devices. However, the low surface tension of uncrosslinked PDMS limits the spatial resolution of the patterns, resulting in broad and irregularly shaped barrier wall structures.
A new two-step method for patterning straight barrier walls was proposed: hydrophobize the entire paper substrate with Alkyl Ketene Dimer (AKD), and then create hydrophilic channels via a plasma patterning process. Although both PDMS-and AKD-based LOPs are flexible, the channels are relatively wide (1-2 mm) because of the patterning limitations. Controlled fabrication of channels with widths of several hundred micrometers has been achieved by printing hydrophilic patterns via inkjet printing.
The use of widely available technology to design LOP devices, for example a standard desktop printer, clearly offers substantially enhanced versatility, since it enables end-users to “program” LOP devices according to specific needs. A recent report has noted that programmable LOCs would be the next critical innovation in this technology. Most current LOP technologies limit the ability of non-expert users to program their own devices because of the complex chemicals, methods, and/or equipment needed for device fabrication.
Furthermore, all the LOP concepts discussed above depend on absorption of test fluids into the hydrophilic areas of porous paper and use capillary forces for fluid actuation. As a result, the fluid that has been incorporated inside a LOP cannot easily be extracted for further biochemical analysis. This is particularly important because the analysis in LOPs is currently semi-quantitative at best; the accuracy and sensitivity cannot compete with traditional analytical equipment.
One option to overcome this particular issue is to prevent absorption of the liquids into the paper matrix. By restricting droplets to the surface of the substrate, the samples are accessible for post-processing and quantitative analysis in a centralized testing center, while simple qualitative biochemical characterizations can still be performed at the point-of-care (POC).
In order to achieve this, droplets must be manipulated on a two dimensional substrate that enables basic unit operations: storage, guided transport, mixing and sampling. Two-dimensional microfluidic lab-on-chip devices have been previously obtained via electrowetting and optoelectrowetting (OEW), but these approaches require external power sources for operation and complicated fabrication methods. Ideally, a two-dimensional LOP should be inexpensive, enable design flexibility and operate without an external power source.
Thus, as discussed above, it would be desirable to pattern surfaces to create regions of variable adhesive force on a single superhydrophobic paper surface. It is to such systems and methods that that present invention is primarily directed.